This invention relates generally to electric motors and, more particularly, to a circuit for reducing voltage stresses, or surges, between a cable and an electric motor.
Voltage stresses and surges are generally recognized as undesirable for electric motors for a variety of reasons. For example, surges often present voltages to a motor having a high amplitude and steep wave front which can damage supply line insulation or damage a stator winding's first coil.
The steep wave front can lead to non-uniform distribution of the increased voltage through the winding. Generally the first coil, which is connected to the motor supply lead, takes the largest portion of the voltage. The non-uniform distribution of the voltage through the first coil can lead to premature motor failure.
Electric motors may include an inverter, such as a pulse width modulated (PWM) inverter, to supply a series of pulses to the motor. These pulses travel along the transmission line to the motor and can have steep wave fronts. Since the motor generally has a higher surge impedance than the transmission line, a reflection may occur which causes the voltage at the motor to increase by a ratio of 2 Z.sub.m /(Z.sub.m +Z.sub.0) where Z.sub.m is the surge impedance of the motor and Z.sub.0 is the surge impedance of the transmission line. The motor impedance may be many times the surge impedance of the line.
Accordingly, it would be desirable to reduce, or eliminate, the reflection of the pulse along the transmission line.